What happens Vout on the PCM17xx if BCK starts and stops?

Jamaal_Charles

Other Parts Discussed in Thread: PCM1742, DAC1282

Hello,

We are looking to use a handful (4 or so) PCM17xx's in our non-audio application. The number of bits and specs are what we need. Our thought is to use a micro with an I2S port (maybe an M4F), and then mux it through a CPLD or FPGA to multiple DAC's. Is there an app note for using multiple DAC's? I see in some of the datasheets where multiple channels are used.

Our main question is what will happen to the part if BCK is not always on. In other words, we will be muxing it to the other DAC's, so we are wondering what happens to Vout if BCK isn't always running? We are hoping it won't go to zero, and we could basically set it to an output voltage like a typical DAC, and then program the next DAC to another voltage level.

Thanks,

Jamaal

over 11 years ago

0 LeonardEllis over 11 years ago

TI__Guru 71526 points

I took the PCM1742 as an example. Page 12 describes the Audio Serial Interface, and from what is stated, it seems the VOUT is going to be at the bipolar zero level of Vcc/2, but only after BCK is reintroduced (and assuming LRCLK and SCK also present). During the period where BCK is inactive, the assumption is that VOUT remains at its last value:

Accordingly, internal operation of the device is suspended when the sampling rate clock of LRCK is changed or SCK and/or BCK is interrupted at least for three bit-clock cycles. If SCK, BCK, and LRCK are provided continuously after this suspended state, the internal operation is resynchronized automatically within a period of less than 3/fS. During this resynchronization period and for a 3/fS time thereafter, the analog output is forced to the bipolar zero level, or VCC/2.

From this description, I am not absolutely sure this is the case, so I'll get with the product group to find out.

-Leonard

0 Dominik Hartl11 over 11 years ago in reply to LeonardEllis

TI__Guru 55556 points

The output is not guaranteed to stay constant when the clocks are stopped or changed.

0 LeonardEllis over 11 years ago in reply to Dominik Hartl11

TI__Guru 71526 points

What would it typically do (or can do)? What are the constraints?

0 Jamaal_Charles over 11 years ago in reply to LeonardEllis

Genius 3245 points

Also, this is a pretty basic question, but I assume the DAC outputs are DC and not AC, correct?

0 LeonardEllis over 11 years ago in reply to Jamaal_Charles

TI__Guru 71526 points

As best I can tell, it seems the OUT would remain at the last value set, or, it would turn OFF, meaning, HiZ. For sure, not AC, as this would have to be driven by the serial data. I don't know if this situation was ever characterized, as it is not a normal operating mode, however, it is a real-world possibility, so I'll pursue to see if we can get a definitive answer.

-Leonard

0 Don Dapkus over 11 years ago in reply to LeonardEllis

TI__Guru**** 189399 points

Guys,

I would recommend you look at the PCM51xx family. It has very graceful loss of clock operation.

-d2

0 LeonardEllis over 11 years ago in reply to Jamaal_Charles

TI__Guru 71526 points

Might want to look at the DAC1282. This is a Prec Analog 24-bit voltage-out DAC, but SPI interface, not i2C . . . should be much easier to interface than the audio DAC: http://www.ti.com/product/dac1282 .

0 Dafydd Roche over 11 years ago in reply to LeonardEllis

TI__Expert 5555 points

I second Leonards suggestion.

The PCM parts really aren't designed to be "set and forget" style DAC's. Their DC performance is not specified or guaranteed, as 99.99% of audio applications will have DC blocking caps on both inputs and outputs.

If you really want a DAC that you can "set and forget", the industrial DAC's are designed for this purpose.

best regards

Dafydd Roche

Audio

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What happens Vout on the PCM17xx if BCK starts and stops?